1. Field of the Invention
The present invention relates to an apparatus for recovering thermal energy from an engine which is operated by burning fuel, and more particularly to such a thermal energy recovery apparatus for recovering exhaust energy of an engine.
2. Description of the Related Art
There have been developed in recent years thermally insulated engines having various components made of thermally insulating materials, such as ceramics. These components include, for example, outer wall of an exhaust manifold, cylinder liners, insulating plates for a cylinder head, and pistons. In such a thermally insulated engine, it is not necessary to cool the engine by radiating the heat generated inside the engine. Rather, the energy of high-temperature exhaust gas discharged by the engine is recovered and fed back to the output shaft of the engine for increasing the output power from the engine.
A so-called turbo compound engine has been proposed as an exhaust energy recovery apparatus. According to the turbo compound engine, a turbocharger is operated by the exhaust energy from an engine to supercharge the engine, and an exhaust turbine is driven by exhaust gas having residual energy which is discharged from the turbocharger. Rotary power from the exhaust turbine is fed back to the output shaft of the engine.
However, the turbo compound engine of the type described above requires a speed reducer having a large speed reduction ratio in order to allow the rotational speed of the exhaust turbine, which is rotated at high speed by the exhaust energy, to match the rotational speed of the engine. Therefore, such an exhaust energy recovering apparatus is large in size, and the power transmission efficiency is less than the efficiency desired.
To solve the problems of the above turbo compound engine, the inventor has proposed an exhaust energy recovery apparatus as disclosed in Japanese patent application No. 60-233938. The exhaust energy recovery apparatus proposed therein includes a generator driven by a turbine disposed in an exhaust pipe, another generator driven by another turbine disposed at the exhaust outlet of the first turbine, and a motor coupled to the drive shaft of a motor vehicle. The two generators are controlled according to the operating condition of the motor vehicle, for driving the motor to give assistive running power to the motor vehicle.
However, the above proposed energy recovery apparatus still has drawbacks. It fails to recover exhaust energy in a wide range of engine operating conditions, since where exhaust energy is recovered according to the depth to which an accelerator pedal is depressed, it is recovered dependent on the ratio between an engine intake pressure and a boost pressure.